1. Field of the Invention
The present invention relates generally to the field of precision cleaning systems. More specifically, the present invention discloses a spray cleaning system for use with a flammable solvent having safety features to maintain the concentration of solvent vapor in the cleaning chamber below a safe limit. In addition, the present system safely confines, extinguishes, and/or dissipates any fire or explosion within the cleaning chamber.
2. Statement of the Problem
Precision cleaning systems have been used for many years in fields where component surfaces must be cleaned to very fine cleanliness levels. For example, precision cleaning is frequently used in the aerospace, semiconductor, and biomedical industries. The cleaning process usually involves a spray of solvent directed against the surfaces of the workpiece to dislodge particulates and remove grease and other molecular contaminants. Conventional precision cleaning systems have commonly used any of a number of chlorofluorocarbon (CFC) compounds as the cleaning solvent. These CFC compounds are effective cleaning agents and are essentially chemically inert. In particular, CFC compounds are not flammable or explosive. Therefore, conventional precision cleaning systems have not had to deal with the possibility of combustion or explosion of the solvent spray.
CFC compounds have increasingly been the subject of environmental concerns in recent years. It is believed that CFC compounds may be a cause of ozone depletion in the upper atmosphere. As a result, production of CFC compounds is being phased out by the chemical industry. It is anticipated that commercial supplies of CFC compounds will become increasing difficult to obtain over the next several years. Efforts have therefore been undertaken to identify other compounds that can be used as solvents in precision cleaning systems.
One possible alternative is to use an alcohol, such as isopropyl alcohol, as the solvent. Alcohol has been used in the past as a solvent or cleaning agent. But, the prior art systems involve immersion of the workpiece in a bath of alcohol. Although some degree of fire risk is associated with using a vat of alcohol as in the prior art, there is a quantum increase in the risk associated with using an alcohol spray to clean the workpiece. The fine mist of alcohol droplets and the level of vapor phase alcohol resulting from spraying can quite easily produce highly flammable or explosive conditions within the cleaning chamber.
A number of cleaning systems have been invented in the past that use solvent sprays and include means to recycle used solvent, including the following:
______________________________________ Inventor Pat. No. Issue Date ______________________________________ Rand 4,101,340 July 18, 1978 Blaul 4,561,903 Dec. 31, 1985 Blaul 4,886,081 Dec. 12, 1989 Blaul 4,676,261 June 30, 1987 Capella, et al. 4,443,269 Apr. 17, 1984 Vetter 4,688,585 Aug. 25, 1987 Rudolphi 4,187,868 Feb. 12, 1980 Fowler, et al. 4,865,061 Sep. 12, 1989 Petit, et al. 4,940,494 July 10, 1990 Blackwood 4,132,567 Jan. 2, 1979 Cherry 4,936,922 June 26, 1990 Cherry, et al. 4,832,753 May 23, 1989 Roberson, et al. 4,977,688 Dec. 18, 1990 Weil 5,011,542 Apr. 30, 1991 Koblenzer 4,390,396 June 28, 1983 Hughes 4,993,199 Feb. 19, 1991 ______________________________________
Rand discloses a solvent spray cleaning system for minimizing solvent losses. A treatment chamber has a spray cleaning zone and a sump separated by a wall and a closure member so that the spray cleaning zone can be isolated from the sump. During this isolation, solvent vapors are removed from the cleaning chamber to facilitate introduction and removal of work into the chamber.
The Blaul patents disclose a method of solvent spray cleaning in an enclosed chamber. A hand-directed spray nozzles directs a high pressure spray of solvent within the enclosed chamber. The internal air of the chamber and the solvent are continually recycled. The system also includes means to keep down the vapor concentration within the chamber by defogging the chamber air.
Capella, et al., disclose a system for cleaning contaminated articles using a high pressure spray gun within a cleaning chamber. The solvent is recycled within the unit.
Vetter discloses an automatic washer for cleaning objects having a housing with an opening for introduction of the object to be cleaned and a number of nozzles that are aligned and oriented appropriately to clean the surfaces of the object. A control device detects and responds to the shape, orientation, and position of the object to control the method and timely running of the cleaning process.
Rudolphi discloses a system for cleaning a surface of an optical element in which a cleaning liquid is applied to the surface and vibrated at a frequency (such as 20-50 kHz) to facilitate cleaning.
Fowler, et al., disclose a system for solvent spray cleaning in an enclosed chamber. The internal air of the chamber and the solvent are continually recycled. A cooling means within the exhaust duct condenses the solvent vaporized during the cleaning operation. The pressure within the chamber is reduced to sub-atmospheric pressure to prevent the possibility of leakage of contaminants from the chamber into the surrounding environment.
Petit, et al., disclose a process for cleaning a large part by submerging the part in a tank of solvent and then subjecting the submerged part to low-frequency ultrasound. The solvent can be recycled for future reuse.
Blackwood discloses an apparatus for cleaning and removing static charges from integrated circuit wafers. The wafers are successively revolved through a spray of deionizing water and drying nitrogen gas in a closed chamber. Ionized nitrogen gas is introduced into the chamber to eliminate static charge on the wafers.
The Cherry patents disclose a system for high purity cleaning in a chamber maintained at a pressure above ambient atmospheric pressure. Components to be cleaned are transported into the cleaning chamber on a gas-glide transporter mechanism. The components are first cleaned using a high pressure spray of individual droplets of solvent and are then dried by filtered air.
Roberson, et al. discloses a device for drying semiconductor wafers with isopropyl alcohol in vapor form.
Weil discloses a system for treating objects in a closed vessel with a solvent. The objects are boiled in a mixture of solvent and water in the vessel.
Koblenzer discloses an apparatus for distillation of solvents, such as alcohol.
Hughes discloses a portable self-contained decontamination booth. An exhaust air system maintains a negative pressure within the work station. The solvent is recycled.
3. Solution to the Problem
None of the prior art references uncovered in the search show a spray cleaning system for use with a flammable solvent, such as isopropyl alcohol, having safety features to maintain the concentration of solvent vapor within the cleaning chamber below a safe limit and to mitigate static discharge. Sensors detect the presence of combustion or an explosion within the cleaning chamber and automatically shut down the flow of solvent to the cleaning chamber. The system also is designed to safely confine, extinguish and/or vent any fire or explosion within the cleaning chamber.